Method and device for determining the direction in which an object is located

ABSTRACT

The invention concerns a method and device for determining the direction of an object ( 10 ) which emits or reflects optical radiation. An optical element ( 14 ) which structures the wavefront of the radiation converts the conventional dot-type image ( 20 ) into an intensity distribution ( 14 ′) with more than one maximum on a locally resolving opto-electronic detector ( 13 ). From the intensity distribution ( 14 ′) and the structure function of the optical element ( 14 ), the direction of the object ( 10 ) can be determined with a high degree of accuracy over a wide measurement range. In addition, this gives a design with a shorter optical path length and very few optical components.

The invention relates to a method and a device for determining thedirection, defined by a horizontal and vertical angle, to an objectwhich emits or reflects optical radiation, the radiation being picked upby an imaging optical system for producing an object image on aspatially resolving optoelectronic detector, the detector signals beingfed to an evaluation device, and the direction to the object beingdetermined from the coordinates of the object image on the detector.

The radiation coming from the object is intended to lie in the opticalwavelength region and to be emitted, scattered or reflected by theobject.

DE 32 33 013 A1 has disclosed an optical arrangement with the aid ofwhich it is possible to detect and evaluate the spatial position of a3-dimensional object by means of edge detection, and to determine thedistance from the object. This arrangement can be applied chiefly inimage evaluation systems for automatic manipulators. Two seriallyarranged lenses with different diaphragm openings, a raster filter and atelevision camera are used to detect the position of the threedimensions. Specific geometrical relationships are maintained bothbetween these subassemblies inside the optical arrangement and inaddition to the object to be imaged, in order to satisfy the imagingequations.

The raster filter comprises spatially distinctive, periodic structuressuch as, for example, sinusoidally cambered surfaces, lenses,cylindrical lenses or prisms with a prescribed bevel, or prism elementsarranged in a crossed fashion. These structures make use of therefraction of the light beams, which are thereby deflected by a specificangle α and thus displace the object points to be imaged. The brightnesstransitions at the edges of the object are periodically modulated by theperiodic structures of the raster filter. They are picked up line byline by the television camera, and the signals are electronicallyevaluated. The periodic disturbances of the edge image contain in a formcoded by the raster filter the information on the rotary position andthe course of the object contours. The distance from the object islikewise derived, on the basis of the existing functional relationships,from the superimposition of the imaged object contours with the rasterstructure of the raster filter.

The use of refractive, spatial structures for the raster filter, as wellas the use of two lenses and the maintenance of a series of geometricalconditions signify a comprehensive outlay on production and adjustment.Recording by the television camera is performed with the resolutionprescribed by the video scanning.

CH 665 715 A5 has disclosed a method for measuring the angulardisplacement of an object by means of an object-referred aiming markerdesigned as an optically structured scanning disk. In this case, animage of the aiming marker is photoelectrically evaluated, and theinformation thus obtained is compared with a reference markercorresponding to the aiming marker. In detail, the aiming marker, whichis provided with a defined center, is imaged onto the detection plane ofa detector array, whereupon the values thus obtained are subjected to acorrelation comparison with the reference marker. The displacement atthe center of the aiming marker from the optical axis is then calculatedfrom the result of the comparison. The available resolution of thestructure and position of the object is given in this known method firstand foremost by the individual detection ranges of the detector array,that is to say discrete pixel geometry, and correspondingly limited .

Under the title of “Theodolitsysteme fär industrielle und geodätischeMessungen” [“Theodolite systems for industrial and geodeticmeasurements”], there is a description on pages 14 to 18 of the journalentitled Technische Rundschau No. 39, 1988 by W. Huep and O. Katowski oftheodolite systems which are used for contactless measurement ofsurfaces such as, for example, claddings of aircraft or body parts withthe aid of reflecting aiming markers. In this case, a search lightarranged coaxially with the axis of the theodolite telescope illuminatesan aiming marker which is imaged by the theodolite telescope on a CCDarray as a spatially resolving detector. An electronic evaluation devicewith a computer determines the center point of the aiming marker image.The horizontal and vertical angle of the aiming marker are determined ina prescribed coordinate system from the coordinates of the center pointof the aiming marker image on the CCD array.

Surface-reflecting spheres, for example chromized, polished steelspheres, which present the same aiming marker image in each caseirrespective of the direction of observation serve as aiming markers.The reflecting spheres produce a virtual image, situated in the interiorof the sphere, of the search light pupil of the theodolite, which isobserved with the telescope of the theodolite and represented on the CCDarray. Because of the short focal length of the spheres, however, thepupil image in the sphere is already small, as a result of limitation bydiffraction, given a short distance between the theodolite and sphere,and it is smaller on the CCD array than the pixel size thereof. In orderfor it to be possible to take any picture at all, the theodolitetelescope is defocused so as to produce a light spot which can be pickedup by a plurality of pixels of the CCD array. The center point of thelight spot thus obtained is determined by center or contour evaluation.However, because of the defocusing the different intensity distributionof the radiation in the light spot and its fuzzy edge leads to measuringerrors.

In general, of course, it is possible to use lenses with a large imagescale or an image scale which can be varied for range adjustment, inorder to obtain a sufficiently large image on the CCD array. As aresult, said defocusing of the theodolite telescope in order to producea sufficiently large light spot diameter could be eliminated, forexample. However, a large image scale entails the use of acorrespondingly large lens focal length. Special telescope lenses orcollimators have focal lengths of 2 m and more for this purpose. In thiscase, the range of angular measurement is necessarily substantiallyrestricted for the same detector size. In addition, collimators withsuch focal lengths produce large-volume optical instruments of highweight.

It is the object of the invention to specify a method and a device bymeans of which it is possible, with a very low outlay on opticalcomponents and adjustments and in conjunction with drastically shortenedmechanical overall lengths, to determine the direction of the opticalradiation coming from an object within a large range of angularmeasurement, the aim being to achieve a precision which goes far beyondthe spatial resolving power, conditioned by the design, of anoptoelectronic detector to be used.

According to the invention, this object is achieved by virtue of thefact that an optical element is used to structure the wavefront of theradiation coming from the object in such a way that an intensitydistribution with more than one intensity maximum is produced on thedetector, and in that the direction to the object is determined from themeasured intensity distribution by making use of the structure functionof the optical element. Furthermore, the object is achieved by means ofthe features specified in the characterizing part of the device claim 3.

Advantageous developments and improvements of the invention follow fromthe features of the subclaims.

The radiation emitted by a point light source propagates spherically inall spatial directions in a homogeneous medium. This means that thesurfaces of the same phase are spherical surfaces and expand. Aspherical surface is virtually flat when seen at a large distance fromits center point and in a small section. If the opening of an imagingoptical system corresponds to this section, said opening receives anapproximately plane wave or—seen in the beam image—virtually parallelbeams.

The imaging optical system, which in the simplest case can berepresented by a collective lens, focuses such radiation in its focalplane. Consequently, a point light source which is far removed appearsin a punctiform fashion in the focal plane of the imaging opticalsystem. The same also holds for an approximately punctiform radiationspot produced by projection of a radiation source on a remote object.Radiation emitted by a radiation source and shaped by an optical deviceto form a parallel beam is likewise imaged by the imaging optical systemin a punctiform fashion in its focal plane. In this case, the parallelbeam can reach the imaging optical system directly or via the path of areflection at a flat object.

Given a sufficient spatial resolving power, a spatially sensitivedetector arranged in the focal plane of the imaging optical systemrecords the punctiform image as an intensity maximum at a specificlocation. This location on the detector depends on the angle which theparallel beam forms with the optical axis of the imaging optical system.With increasing angle, the punctiform image moves away from the opticalaxis. In the case of three-dimensional viewing, and with a detectorwhich is spatially sensitive in two dimensions, each coordinate point onthe detector is uniquely assigned a direction of the incident radiation,the direction being given by a horizontal and vertical angle. Thus, ifthere is a change in direction of the incident radiation owing to achange in one or both of these angles, there is a consequent change inthe coordinates of the radiation point on the detector surface.

This description also holds for problems where it is desired to measureonly one angle in a fixed plane. Specifying a second angle is notimportant for this purpose; it can be set arbitrarily to zero. Adetector which is spatially sensitive in one dimension, such as a lineardiode cell, for example, can also be used for such a case.

In addition to the plane waves previously considered, it is alsopossible, of course, for the imaging optical system to record sphericalwaves. In such a case, the object is located in the near zone of theimaging optical system, and emits or reflects punctiform radiation. As aresult, it is necessary for the imaging optical system to be sharplyfocused, the object being imaged in the image plane of the imagingoptical system. For the purpose of determining direction, it is thennecessary in this case, of course, also to take account of the imagedistance set by the imaging optical system in addition to thecoordinates of the image point on the detector.

According to the invention, there is brought into the beam path anoptical element which structures the wavefront of the radiation comingfrom the object in such a way that instead of the conventionalpunctiform object image, an intensity distribution with more than oneintensity maximum is produced on the detector. The optical element canfundamentally be arranged in this case at any point on the beam path.However, it is preferably arranged in the vicinity of or directly in theplane of the exit pupil, or in a pupil plane of the imaging opticalsystem which is conjugate thereto. Of course, a conjugate pupil planecan also be situated upstream of the imaging optical system, that is tosay between the imaging optical system and the object, it being possiblefor the optical element structuring the wavefront also to be arrangedthere.

The physical effect of the optical element on the wavefront of theradiation can be described mathematically in a unique way by thestructure function of the optical element, which is determined by thestructure of the optical element. The structure function acts on thewavefront of the radiation and leads to a corresponding intensitydistribution on the detector. Because of the uniqueness of thisrelationship, it is possible in reverse sequence to use the measuredintensity distribution and the structure function of the optical elementto reconstruct the incident wavefront, and thus the direction of theorigin thereof, that is to say the direction of the radiation comingfrom the object. It is also possible, in principle, for the result of apreceding calibration to be used instead of the structure function ofthe optical element.

Various optical properties can be utilized to realize such an opticalelement structuring the wavefront. Thus, for example, the wavefront ofthe radiation can be spatially influenced by refractive, reflective orpolarizing properties, or also by the effect of diffraction.

When refractive structures are used, the optical element ischaracterized by refractive indices or material thicknesses which differas a function of location. In order to achieve a desired intensitystructure on the detector, the spatial wavefront modulation can beappropriately varied by means of the refractive index function and/orthickness function producing them. The refractive index function andthickness function represent the structure function of the opticalelement.

Regions which polarize differently as a function of location can be usedin a similar way. Dichroitic materials yield linear polarization which,depending on the alignment of these materials lead [sic] to differentpolarization directions as a function of the location. In general,different elliptical states of polarization are also possible, and theseare produced, for example, with the aid of spatially varying thicknessof birefringent materials. With the aid of an analyzer, a correspondingintensity distribution is produced on the detector from the spatialpolarization modulation, which in this case represents the structurefunction of the structuring optical element.

The optical element according to the invention can also be representedwith the aid of diffractive structures. For this purpose, it is possibleto use Fresnel zone plates or holographic elements, for example, in thecase of which the grating structure varies locally. Such diffractivestructures can be produced on a carrier by various lithographic, etchingor vapor-deposition methods, or by embossing, bright pressing ormilling. In this case, the carrier can be transmitting or reflecting forthe radiation, that is to say the optical element can be used totransmit or reflect.

Finally, it is also possible to utilize for the optical element all thephysical effects which produce a spatial modulation, conditioned by thestructure of the optical element, of the wavefront from the incidentradiation, doing so in such a way that an intensity distribution withmore than one intensity maximum is produced on the detector. Theintensity distribution can also be regarded as an intensity pattern oras a code. In the event of a change in direction of the incidentradiation, the intensity pattern or the code is displaced on thedetector.

The evaluation of the measured intensity distribution is performed, forexample, using the known methods of averaging or by fit algorithms. Theysupply a substantially improved accuracy in the measurement of directionby comparison with the evaluation of an individual radiation point,because evaluating the position of an extended structure leads to abetter signal-to-noise ratio. In addition, there is a substantialreduction in the risk of overexposing the detector, which is otherwisealways present in the case of focusing the incident radiation onto onlya single radiation point. Moreover, the evaluation of an intensitydistribution is more reliable, because disturbances in the beam path areeasily compensated by measuring a multiplicity of radiation points ofthe intensity distribution.

A particularly improved spatial resolving power, and thus a particularmeasurement sensitivity for determining direction is yielded by anintensity distribution whose spatial fundamental frequency or one of itsspatial harmonic frequencies forms with the spatial fundamentalfrequency of the radiation-sensitive structures of the detector alow-frequency heterodyne pattern. The low-frequency heterodyne patternacts in the same way as a moire pattern. It is known that moire patternsreact very sensitively to a displacement of the structures producingthem. This means here that even a very slight displacement of theintensity distribution on the detector with respect to its pixelstructure produces a strong change in the spatial frequency of thelow-frequency heterodyne pattern. The change in the heterodyne patternis therefore a very sensitively reacting indicator of changes in theintensity distribution on the detector. Spatial information cantherefore be resolved more effectively by a factor of more than 100 thanwould be possible from the geometry of the pixel structure of thedetector. The result of this is a correspondingly high accuracy in thedetermination of angle and direction relative to the object.

In addition to raising the accuracy and the robustness of directionalmeasurement, the production of a structured light distribution on thedetector is attended further by the additional advantage of theexpansion of the range of measurement. In the case of a punctiform lightdistribution, the range of directional measurement is determined by thesize of the detector surface. However, in the case of a structuredintensity distribution, it is the extent of the spatial structures whichis decisive. This can be substantially larger than the detector surface.It is possible to determine direction even if a relatively large part ofthe intensity distribution is no longer situated inside the activedetector surface. This is illustrated by the following example.

The diffraction at a simple diffraction grating with rectilinearstructures is sufficient to produce on the detector an intensitydistribution with a principal maximum and a plurality of secondarymaxima arranged on both sides. In this case, the secondary maxima, thatis to say the diffraction maxima of higher order, can overreach theactive detector surface. If, because of a large angle of incidence ofthe radiation, the principal maximum, which corresponds to the originalpunctiform imaging of the radiation source, is no longer situated on theactive detector surface, it can be reconstructed from the position ofthe secondary maxima. The only decisive point is that the secondarymaxima can be identified, and this is possible in principle because oftheir different intensities.

In summary, the advantages of the invention consist in that a highermeasurement accuracy and a wider measurement range for determining thedirection to an object are rendered possible with the aid of the opticalelement structuring the wavefront. A simple imaging optical system witha short focal length suffices because of the substantially improvedspatial resolving power. This results advantageously, on the one hand,in substantially shortened mechanical overall lengths and a lesseroutlay on optical components and adjustments. There is thus animprovement in manipulation and costs. On the other hand, short focallengths permit a wider range of angular measurement relative to theobject. Said range is moreover further substantially widened beyond thesize of the detector because of the expanded intensity distribution. Theresult of this is that it is still possible to determine direction evenif the conventional object image is already located outside theradiation-sensitive surface of the detector.

The following exemplary embodiments of the invention are explained inmore detail with the aid of the drawing, in which:

FIG. 1a shows a diagrammatic representation of an optical elementstructuring the wavefront of radiation in transmission and which isarranged separately between an imaging optical system and a detector,the incoming radiation having plane wavefronts,

FIG. 1b shows a diagrammatic representation as in FIG. 1a, but with theoptical element arranged between the imaging optical system and theradiation source,

FIG. 2 shows a diagrammatic representation in accordance with FIG. 1a,in which the radiation reflected by a rotary mirror is picked up,

FIG. 3 shows a diagrammatic representation in accordance with FIG. 1a,it being the case, however, that the incoming radiation has sphericalwavefronts,

FIG. 4 shows a diagrammatic representation of the subject-matter of theinvention, the structures modulating the wavefront of the radiation in aspatial fashion being applied directly to a surface of a refractiveimaging optical system,

FIG. 5 shows a diagrammatic representation of an optical element whichstructures the wavefront of the radiation and simultaneously has imagingproperties,

FIG. 6 shows a diagrammatic representation of an optical element whichoperates by reflection and structures the wavefront of the radiation,

FIG. 7 shows a diagrammatic representation of the subject matter of theinvention, the structures modulating the wavefront of the radiation in aspatial fashion being applied to a surface of a reflective imagingoptical system, and

FIG. 8 shows an intensity distribution produced with the aid of theoptical element according to the invention.

FIG. 1a shows a diagrammatic representation of an optical element 14which structures the wavefront of radiation and operates bytransmission. The optical element 14 is arranged separately between animaging optical system 12, in this case a lens with a refractive opticalsystem, and a spatially resolving optoelectronic detector 13. Theobserved object 10 here takes the for m of a remote radiation source,the radiation falling into the imaging optical system 12 havingapproximately plane wavefronts 11. The observed object 10 can also be areflecting body which is illuminated by the radiation of a radiationsource or by scattered light, and whose reflected radiation withapproximately plane wavefronts 11 is picked up by the imaging opticalsystem 12.

Without the optical element 14, the incoming plane wavefronts 11 wouldbe imaged by the imaging optical system 12 in a punctiform fashion atthe location 20 on the detector 13. The diameter of this radiation spotis generally small by comparison with the light-sensitive structures ofthe detector 13. The spatial fundamental frequency (scanning frequency)of the light-sensitive detector structures is therefore too small todetect the high spatial frequency components of the punctiform intensitydistribution. If the inventive optical element 14 structuring thewavefront 11 of the radiation is then introduced into the beam path inaccordance with FIG. 1a, the incoming wavefronts 11 are spatiallymodulated in such a way that instead of the punctiform image 20 of theobject 10 an intensity distribution 14′ with more than one intensitymaximum is produced on the detector 13. The detector signals aretransmitted into an evaluation device 17 in which the position of theintensity maxima of the intensity distribution 14′ is determined. Thehorizontal and vertical angles of the incident wavefronts 11, that is tosay the direction to the observed object 10, are determined therefromwith the aid of the structure function of the optical element 14.

This direction can be determined with particular precision when thespatial fundamental frequency or one of the spatial harmonic frequenciesof the intensity distribution 14′ forms a low-frequency heterodynepattern with the spatial fundamental frequency of the detectorstructures. This heterodyne pattern is very sensitive to lateralmovements of the object 10, that is to say to changes in direction ofthe radiation falling into the imaging optical system 12, which effectcorresponding displacements in the intensity distribution 14′ on thedetector 13. As a result, it is also possible to perform a very precisedynamic measurement and to exactly record, measure and track a lateralmovement of the object 10. If, in the process, the object 10 leaves thedetecting range of the arrangement in FIG. 1a, it is possible for theobject 10 to be tracked continuously over the entire range of solidangle by motorizing and automating the arrangement.

A glass plate with a structure which has been etched in serves here asan optical element 14 structuring the wavefront. The optical element 14is thus monolithically integrated in the glass substrate. It is alsopossible to use an amplitude or phase hologram. Other possibilities ofrealization have already been named above. The optical element 14 ispreferably arranged in the vicinity or in the exit pupil of a pupilplane, conjugate thereto, of the imaging optical system 12. Such aconjugate pupil plane can also be situated upstream of the imagingoptical system 12, that is to say between the latter and the object 10.An optical element 14 arranged at this point is shown in FIG. 1b.

The representation in accordance with FIG. 2 shows as observed object arotary mirror 30 which is assembled in this exemplary embodiment from 6plane mirrors. Its rotation axis 31 is arranged in a stationary fashion.A radiation source 5 is likewise arranged in a stationary fashion andilluminates the rotary mirror 30, which reflects the incident radiation.The angle of reflection is a function of the rotary position of therotary mirror 30 in this case. As a result, the reflected radiation hasa direction assigned to the rotary position. A rotary movement of therotary mirror 30 effects a change in direction of the reflectedradiation. The rotary position of the rotary mirror 30 can be determinedwith high precision with the aid of the arrangement represented and ofthe measurement of direction according to the invention.

In the representation according to FIG. 3, the imaging optical system 12receives spherical wavefronts 11 a. The object 10 is thus located in theclose vicinity of the imaging optical system 12. In order to ensureimaging, detector 13 is therefore located in an image plane B. Dependingon the distance between the object 10 and the imaging optical system 12,when focusing is performed there is a change in the image distance b andthus in the position of the intensity distribution on the detector. Thisis to be taken into account when determining the direction.

In the exemplary embodiment according to FIG. 4, the imaging opticalsystem 12 is shown as a plane-convex lens. Structures 16 are applied toits plane surface. The structures 16 resemble those of the opticalelement 14 as mentioned in the description of FIG. 1a. In particular,the structures 16 can also be diffraction structures. They are produced,for example, by etching or vapor deposition or applied as film. Insteadof a punctiform image 20, supplying only a little information, of theincident plane wavefronts 11, the structures 16 produce an intensitydistribution 16′. For the measurement of direction, this results in theadvantages already mentioned of the higher measurement accuracy, thelightening of the measurement range and the compactness of theopto-mechanical design. In addition, in this exemplary embodiment thelow outlay on components is also illustrated, there being finally only asingle optical component.

FIG. 5 shows an optical element 24 which has properties which image withthe aid of its structures 26, and which at the same time structures thewavefront of the incident radiation. Consequently, the structures 26produce from the incident wavefronts 11 an intensity distribution 26′consisting of a plurality of intensity maxima, in conjunction withsimultaneous focusing.

In the exemplary embodiments according to FIGS. 1-5, the opticalelements structuring the wavefront of the radiation operate bytransmission. By contrast, FIG. 6 shows an optical element 28structuring the wavefront which operates by reflection. The samestructures 16 as in the case of the transmissive optical element 14 canbe used with a silvered back of the optical element 28. The structures16 can, however, also have properties which are already inherentlyreflective. The imaging of the radiation is performed by means of therefractive imaging optical system 12.

On the other hand, it is also possible to use a reflective imagingoptical system such as, for example, a concave mirror 29 in accordancewith FIG. 7, to image the wavefronts 11. In this exemplary embodiment,the structures 16, which produce the intensity distribution 16′, areapplied to the inner surface of the concave mirror 29. The principle ofthe mode of operation corresponds to that of the above exemplaryembodiments.

In the case of the use of a detector 13 which is spatially sensitive intwo dimensions, the intensity distribution, such as is produced with theaid of one of the optical elements which structure the wavefront and areshown in FIGS. 1-7, is detected in two dimensions simultaneously. FIG. 8shows one excerpt from this two-dimensional intensity distribution,specifically the variation in the intensity as a function of thecoordinate x for a specific coordinate y of the detector 13. Moreover,it is known that a one-dimensional detector 13 which is spatiallysensitive only in the x coordinate can suffice for detection of thetwo-dimensional position of an intensity distribution. In this case,however, it is necessary to use a special intensity distribution whichis a unique function of the y coordinate. A plane incident wavefront 11will be assumed below by way of example, in order to determine thedirection to the object 10 from such a two-dimensional intensitydistribution detected by a linear or two-dimensional detector 13.

A plane wave coming from a specific reference direction is described bythe electric field strength E=A * exp (ikr), where A is a complexamplitude which describes the temporal variation (of no interest here)and the initial phase of the wave, and k is the wavenumber vector and ris the space vector, which are in the complex exponent of the efunction. The wavenumber vector k is perpendicular to the wavefront 11and therefore specifies the direction of the radiation and thereforealso the direction to the object. The electric field strength E ismultiplied by the structure function S, which depends on the spacecoordinates, of the optical element, which is known when the element isproduced. An intensity distribution for the reference direction isyielded after Fourier transformation and, if appropriate, aftercalculation of the Fresnel integral and formation of the square of themodulus.

In visual terms, this intensity distribution is present as an aerialimage in the detector plane. The detected image is produced from thisaerial image by the structures of the optoelectronic detector 13, whichare discrete, that is to say consist of individual pixels. The detectedimage can be calculated from the aerial image by continuing the previouscomputing cycle with the aid of the known detector structures. Thiscalculation can be performed by pointwise integration of the aerialimage, that is to say by integration using each pixel, or else bymathematically equivalent methods.

A lateral movement of the object 10, that is to say a change in thedirection to the object 10, leads to a change in the angle of incidenceof the incident wave and thus, in the simplest case, to a lateral offsetof the detected image. In such a case, the offset of the detected imagewith respect to the image calculated for the reference direction can bedetermined by correlation or by other estimating algorithms. Since therelationship is known between this offset and the horizontal andvertical angle of incidence of the radiation relative to the referencedirection, these angles of incidence are finally determined asdirectional coordinates of the object 10.

In other cases, for example when low-frequency heterodyne patterns areformed by superimposing the spatial frequencies present in the aerialimage with the spatial frequency of the detector 13, there is a morecomplex relationship between the aerial image and the detected image. Anoffset of the aerial image additionally leads to a change in the shapeof the detected image. In such a case, the interaction between theaerial image and the radiation-sensitive structures of the detector 13must be taken into account in an estimating algorithm.

Moreover, it is also possible to determine the direction to an extendedobject 10 which itself has structures. When such an object 10 is imaged,the object image naturally also contains the corresponding structures ofthe object 10, and this is enough to produce an intensity distributionon the detector 13 without an optical element structuring the wavefront.However, such an intensity distribution is structured in a substantiallyeven finer fashion by the optical element, as is illustrated by theintensity distribution in accordance with FIG. 8. With the multiplicityof intensity maxima, this can therefore also be declared to be anintensity pattern or a code. The code is additionally impressed on thepure object image. Of course, in the case of an extended object 10provided with structures it is necessary to know its structure function,which likewise features in the computing cycle described.

What is claimed is:
 1. A method for determining a direction, defined bya horizontal and vertical angle, to an object which emits or reflectsoptical radiation, comprising: receiving the radiation by an imagingoptical system for producing an object image on a spatially resolvingoptoelectronic detector; communicating detector signals to an evaluationdevice; determining the direction to the object from coordinates of theobject image on the detector; and structuring a wavefront of theradiation coming from the object with an optical element such that anintensity distribution with more than one intensity maximum is producedon the detector; and further determining the direction to the objectfrom a measured intensity distribution on the basis of the wavefrontstructuring by the optical element.
 2. The method according to claim 1,wherein a spatial fundamental frequency or a spatial harmonic frequencyof the intensity distribution produced by the optical element on thedetector forms a low-frequency heterodyne pattern with the spatialfundamental frequency of radiation-sensitive structures of the detector.3. A device for determining a direction, defined by a horizontal andvertical angle, to an object which emits or reflects optical radiation,comprising: an imaging optical system for producing an object image; aspatially resolving optoelectronic detector for detecting the objectimage; an evaluation device; and an optical element, arranged in a beampath between the object and the detector, which structures a wavefrontof the radiation coming from the object such that an intensitydistribution with more than one intensity maximum is produced on thedetector, and wherein the direction to the object is determined in theevaluation device from a measured intensity distribution by using thewavefront structuring of the optical element .
 4. The device accordingto claim 3, wherein the optical element structuring the wavefront of theradiation is arranged in a plane of an exit pupil or in a pupil plane ofthe imaging optical system conjugate thereto.
 5. The device according toclaim 3, wherein the optical element structuring the wavefront of theradiation is applied to a surface of the imaging optical system.
 6. Thedevice according to claim 3, wherein the optical element structuring thewavefront of the radiation simultaneously has imaging properties.
 7. Thedevice according to claim 3, wherein the optical element containsstructures which refract differently as a function of location.
 8. Thedevice according to claim 3, wherein the optical element containsdiffraction structures.
 9. The device according to claim 3, wherein theoptical element contains structures which polarize the radiationdifferently as a function of location.
 10. The device according to claim3, wherein the optical element structuring the wavefront of theradiation operates by transmission.
 11. The device according to claim 3,wherein the optical element structuring the wavefront of the radiationoperates by reflection.
 12. A device for determining a direction,defined by a horizontal and vertical angle, to an object which emits orreflects optical radiation, comprising: an imaging optical system forproducing an object image; a spatially resolving optoelectronic detectorfor detecting the object image; an evaluation device; and an opticalelement, arranged in a beam path between the object and the detector,which structures a wavefront of the radiation coming from the objectsuch that an intensity distribution with more than one intensity maximumis produced on the detector, and wherein the direction to the object isdetermined in a time independent manner in the evaluation device from ameasured intensity distribution by using the wavefront structuring ofthe optical element.